This invention relates to circuits for operating fluorescent lamps and, more particularly, to an improved circuit for starting and operating a low-voltage fluorescent lamp from a source of AC line voltage.
With the growing need for saving energy in the home, the significantly greater efficiency of fluorescent lamps, in terms of lumens per watt, as compared to the more commonly used incandescent lamps has spurred engineering development of improved methods for placing fluorescent lamps into more wide-spread residential use. One approach toward accomplishing this end has been to search for improved retrofit systems for the existing incandescent lamp fixtures. More specifically, an object has been to provide an improved incandescent-to-fluorescent adapter having a simplified, lightweight and compact design of reduced cost. The retrofit system would have to permit starting and operation of a low-voltage fluorescent lamp from a conventional 120 volt, 60 cycle source of AC line voltage. With respect to independent safety testing laboratories, such as Underwriter's Laboratories (U.L.), however, concern with fluorescent retrofit systems involves socket voltage, along with the size and weight of the conversion unit. The socket voltage or voltage to ground in any portable lamp fixture is limited, for example, by U.L. standard No. 496 to an RMS voltage of 150 volts. This, in conjunction with a socket weight limitation of 2 1/2 pounds, requires the lamp to be operated in a preheat mode using a glow-bottle starter in conjunction with a 120 volt choke or a 150 volt autotransformer ballast. Another safety concern is the possibility of being able to connect an unballasted fluorescent lamp directly across the 120 volt line. The possibility exists that the lamp will implode when operated without its intervening ballast. This means that with a conventional 120 volt preheat lamp/ballast system, the lamp must be contiguous with the ballast and mounted at the socket in the fixture. This would further aggrevate an already cramped socket airspace.
It would be possible to design the ballast as a plug-through unit located on the floor between the lamp fixture and the wall outlet. However, this would require the lamp to be fused to protect it from misuse, but such a fuse would have attendant disadvantages of cost, bulkiness, and possibly would require a unique size and shape so that the end user could not substitute a higher current fuse and thereby defeat the intended protection.
In addition to the aforementioned disadvantages, the conventional low-voltage preheat systems have exhibited comparatively slow and erratic starting.